Generation of a crust-mantle magma mixture

Magma sources and contamination at Cerro Panizos, central Andes

Michael H Ort, Beatriz L. Coira, Mario M. Mazzoni

Research output: Contribution to journalArticle

67 Citations (Scopus)

Abstract

Cerro Panizos, a large caldera in the central Andes Mountains, produced two large dacitic ignimbrites at 7.9 Ma and 6.7 Ma and many andesitic and dacitic lava flows and domes. The older rhyodacitic Cienago Ignimbrite represents the most silicic magma erupted by the system. The younger, much larger volume dacitic Cerro Panizos Ignimbrite is very crystal-rich. containing up to 50% biotite. plagioclase, and quartz crystals in the pumice. It is weakly zoned, with most of the zoning apparent between two main cooling units. Major and most trace elements show little variation through the Cerro Panizos Ignimbrite, but the small range of composition is consistent with typical fractionation trends. Sr, Nd, and Pb isotopic ratios are very "crustal", with initial 87Sr/87Sr values of 0.71 1 to 0.715, εNd values of -7.5 to -10.2, and nearly invariant Pb isotopic ratios (206Pb/204Pb= 18.85, 207Pb/207Pb= 15.67, and 208Pb/204Pb = 38.80). The limited zonation observed in the Cerro Panizos Ignimbrite is explained by impeded crystal settling due to high crystal content. The magma body was a crystal-liquid mush before ascent to the pre-eruption crustal levels. Crystals formed, but did not separate easily from the magma. Limited fractionation of plagioclase and biotite may have occurred, but the composition was largely controlled by lower crustal MASH processes. AFC modeling shows that the Cerro Panizos magmas resulted from a mixture of roughly equal proportions of late Miocene mantle-derived basalts and melts from ∼ 1.0 Ga (Grenville age) lower crust. This occurred in a MASH zone in the lower crust, and set the crustal isotopic ratios observed in the Cerro Panizos magmas. The great thickening of the crust beneath the central Andes Mountains sent upper and middle crustal rock types to lower crustal (and deeper) depths, and this explains the "upper crustal" isotopic signatures of the Cerro Panizos rocks. Minor upper crustal assimilation of early Miocene volcanic or subvolcanic rocks produced much of the isotopic variation seen in the system. The nearly invariant high Pb isotopic values and high Pb concentrations indicate that Pb came almost entirely from the crustal source, and was little altered by any subsequent upper crustal assimilation. This Pb signature is isotopically similar to that of the southern Bolivian Tin Belt, suggesting a widely distributed Pb source. The great difference between compositions of Miocene and Quaternary central Andean volcanic rocks is explained by crustal thickening in early Miocene time leading to abundant lower crustal water and associated fluxed melting during the time of the earlier eruptions. The lower crust dried out considerably by Quaternary time, so less crustal component is present.

Original languageEnglish (US)
Pages (from-to)308-322
Number of pages15
JournalContributions to Mineralogy and Petrology
Volume123
Issue number3
StatePublished - 1996

Fingerprint

igneous rocks
magma
ignimbrite
crusts
Earth mantle
contamination
Contamination
crystal
crust
Andes Mountains (South America)
mantle
Crystals
rocks
isotopic ratio
Miocene
lower crust
Rocks
biotite
assimilation
plagioclase

ASJC Scopus subject areas

  • Geochemistry and Petrology
  • Geophysics

Cite this

Generation of a crust-mantle magma mixture : Magma sources and contamination at Cerro Panizos, central Andes. / Ort, Michael H; Coira, Beatriz L.; Mazzoni, Mario M.

In: Contributions to Mineralogy and Petrology, Vol. 123, No. 3, 1996, p. 308-322.

Research output: Contribution to journalArticle

@article{a6818cb106044264a41cd5b9b44de1e0,
title = "Generation of a crust-mantle magma mixture: Magma sources and contamination at Cerro Panizos, central Andes",
abstract = "Cerro Panizos, a large caldera in the central Andes Mountains, produced two large dacitic ignimbrites at 7.9 Ma and 6.7 Ma and many andesitic and dacitic lava flows and domes. The older rhyodacitic Cienago Ignimbrite represents the most silicic magma erupted by the system. The younger, much larger volume dacitic Cerro Panizos Ignimbrite is very crystal-rich. containing up to 50{\%} biotite. plagioclase, and quartz crystals in the pumice. It is weakly zoned, with most of the zoning apparent between two main cooling units. Major and most trace elements show little variation through the Cerro Panizos Ignimbrite, but the small range of composition is consistent with typical fractionation trends. Sr, Nd, and Pb isotopic ratios are very {"}crustal{"}, with initial 87Sr/87Sr values of 0.71 1 to 0.715, εNd values of -7.5 to -10.2, and nearly invariant Pb isotopic ratios (206Pb/204Pb= 18.85, 207Pb/207Pb= 15.67, and 208Pb/204Pb = 38.80). The limited zonation observed in the Cerro Panizos Ignimbrite is explained by impeded crystal settling due to high crystal content. The magma body was a crystal-liquid mush before ascent to the pre-eruption crustal levels. Crystals formed, but did not separate easily from the magma. Limited fractionation of plagioclase and biotite may have occurred, but the composition was largely controlled by lower crustal MASH processes. AFC modeling shows that the Cerro Panizos magmas resulted from a mixture of roughly equal proportions of late Miocene mantle-derived basalts and melts from ∼ 1.0 Ga (Grenville age) lower crust. This occurred in a MASH zone in the lower crust, and set the crustal isotopic ratios observed in the Cerro Panizos magmas. The great thickening of the crust beneath the central Andes Mountains sent upper and middle crustal rock types to lower crustal (and deeper) depths, and this explains the {"}upper crustal{"} isotopic signatures of the Cerro Panizos rocks. Minor upper crustal assimilation of early Miocene volcanic or subvolcanic rocks produced much of the isotopic variation seen in the system. The nearly invariant high Pb isotopic values and high Pb concentrations indicate that Pb came almost entirely from the crustal source, and was little altered by any subsequent upper crustal assimilation. This Pb signature is isotopically similar to that of the southern Bolivian Tin Belt, suggesting a widely distributed Pb source. The great difference between compositions of Miocene and Quaternary central Andean volcanic rocks is explained by crustal thickening in early Miocene time leading to abundant lower crustal water and associated fluxed melting during the time of the earlier eruptions. The lower crust dried out considerably by Quaternary time, so less crustal component is present.",
author = "Ort, {Michael H} and Coira, {Beatriz L.} and Mazzoni, {Mario M.}",
year = "1996",
language = "English (US)",
volume = "123",
pages = "308--322",
journal = "Contributions of Mineralogy and Petrology",
issn = "0010-7999",
publisher = "Springer Verlag",
number = "3",

}

TY - JOUR

T1 - Generation of a crust-mantle magma mixture

T2 - Magma sources and contamination at Cerro Panizos, central Andes

AU - Ort, Michael H

AU - Coira, Beatriz L.

AU - Mazzoni, Mario M.

PY - 1996

Y1 - 1996

N2 - Cerro Panizos, a large caldera in the central Andes Mountains, produced two large dacitic ignimbrites at 7.9 Ma and 6.7 Ma and many andesitic and dacitic lava flows and domes. The older rhyodacitic Cienago Ignimbrite represents the most silicic magma erupted by the system. The younger, much larger volume dacitic Cerro Panizos Ignimbrite is very crystal-rich. containing up to 50% biotite. plagioclase, and quartz crystals in the pumice. It is weakly zoned, with most of the zoning apparent between two main cooling units. Major and most trace elements show little variation through the Cerro Panizos Ignimbrite, but the small range of composition is consistent with typical fractionation trends. Sr, Nd, and Pb isotopic ratios are very "crustal", with initial 87Sr/87Sr values of 0.71 1 to 0.715, εNd values of -7.5 to -10.2, and nearly invariant Pb isotopic ratios (206Pb/204Pb= 18.85, 207Pb/207Pb= 15.67, and 208Pb/204Pb = 38.80). The limited zonation observed in the Cerro Panizos Ignimbrite is explained by impeded crystal settling due to high crystal content. The magma body was a crystal-liquid mush before ascent to the pre-eruption crustal levels. Crystals formed, but did not separate easily from the magma. Limited fractionation of plagioclase and biotite may have occurred, but the composition was largely controlled by lower crustal MASH processes. AFC modeling shows that the Cerro Panizos magmas resulted from a mixture of roughly equal proportions of late Miocene mantle-derived basalts and melts from ∼ 1.0 Ga (Grenville age) lower crust. This occurred in a MASH zone in the lower crust, and set the crustal isotopic ratios observed in the Cerro Panizos magmas. The great thickening of the crust beneath the central Andes Mountains sent upper and middle crustal rock types to lower crustal (and deeper) depths, and this explains the "upper crustal" isotopic signatures of the Cerro Panizos rocks. Minor upper crustal assimilation of early Miocene volcanic or subvolcanic rocks produced much of the isotopic variation seen in the system. The nearly invariant high Pb isotopic values and high Pb concentrations indicate that Pb came almost entirely from the crustal source, and was little altered by any subsequent upper crustal assimilation. This Pb signature is isotopically similar to that of the southern Bolivian Tin Belt, suggesting a widely distributed Pb source. The great difference between compositions of Miocene and Quaternary central Andean volcanic rocks is explained by crustal thickening in early Miocene time leading to abundant lower crustal water and associated fluxed melting during the time of the earlier eruptions. The lower crust dried out considerably by Quaternary time, so less crustal component is present.

AB - Cerro Panizos, a large caldera in the central Andes Mountains, produced two large dacitic ignimbrites at 7.9 Ma and 6.7 Ma and many andesitic and dacitic lava flows and domes. The older rhyodacitic Cienago Ignimbrite represents the most silicic magma erupted by the system. The younger, much larger volume dacitic Cerro Panizos Ignimbrite is very crystal-rich. containing up to 50% biotite. plagioclase, and quartz crystals in the pumice. It is weakly zoned, with most of the zoning apparent between two main cooling units. Major and most trace elements show little variation through the Cerro Panizos Ignimbrite, but the small range of composition is consistent with typical fractionation trends. Sr, Nd, and Pb isotopic ratios are very "crustal", with initial 87Sr/87Sr values of 0.71 1 to 0.715, εNd values of -7.5 to -10.2, and nearly invariant Pb isotopic ratios (206Pb/204Pb= 18.85, 207Pb/207Pb= 15.67, and 208Pb/204Pb = 38.80). The limited zonation observed in the Cerro Panizos Ignimbrite is explained by impeded crystal settling due to high crystal content. The magma body was a crystal-liquid mush before ascent to the pre-eruption crustal levels. Crystals formed, but did not separate easily from the magma. Limited fractionation of plagioclase and biotite may have occurred, but the composition was largely controlled by lower crustal MASH processes. AFC modeling shows that the Cerro Panizos magmas resulted from a mixture of roughly equal proportions of late Miocene mantle-derived basalts and melts from ∼ 1.0 Ga (Grenville age) lower crust. This occurred in a MASH zone in the lower crust, and set the crustal isotopic ratios observed in the Cerro Panizos magmas. The great thickening of the crust beneath the central Andes Mountains sent upper and middle crustal rock types to lower crustal (and deeper) depths, and this explains the "upper crustal" isotopic signatures of the Cerro Panizos rocks. Minor upper crustal assimilation of early Miocene volcanic or subvolcanic rocks produced much of the isotopic variation seen in the system. The nearly invariant high Pb isotopic values and high Pb concentrations indicate that Pb came almost entirely from the crustal source, and was little altered by any subsequent upper crustal assimilation. This Pb signature is isotopically similar to that of the southern Bolivian Tin Belt, suggesting a widely distributed Pb source. The great difference between compositions of Miocene and Quaternary central Andean volcanic rocks is explained by crustal thickening in early Miocene time leading to abundant lower crustal water and associated fluxed melting during the time of the earlier eruptions. The lower crust dried out considerably by Quaternary time, so less crustal component is present.

UR - http://www.scopus.com/inward/record.url?scp=0030438188&partnerID=8YFLogxK

UR - http://www.scopus.com/inward/citedby.url?scp=0030438188&partnerID=8YFLogxK

M3 - Article

VL - 123

SP - 308

EP - 322

JO - Contributions of Mineralogy and Petrology

JF - Contributions of Mineralogy and Petrology

SN - 0010-7999

IS - 3

ER -